This invention relates to an analog-to-digital Converter (hereinafter referred to as an ADC) which is important for an interface between the analog and the digital circuits, and more particularly to an integrating type ADC as used in the field of welfare equipments, home electric apparatus and instrumentation, etc.
2. DESCRIPTION OF THE PRIOR ART
In a prior art ADC, A-D conversion is done by discharging the charge corresponding to an input signal by a discharging circuit after it is charged in a strorage means, and counting the number of clock pulses by a counter during the discharging time T which is equal to a period that a level of the charge reaches a constant detection level V.sub.T. An example is described in an article entitled "An All-MOS Analog to Digital Converter Using a Constant Slope Approach", IEEE Journal of Solid State Circuits, June 1976 by G. Smarandoin et. al.
However, in such a circuit construction of the prior art, if an input signal voltage is less than the detection level V.sub.TH, the discharge time T cannot be detected, and thus A-D conversion is impossible. In another word, in the prior art circuit, the A-D conversion is possible in a limited range of the input signal.